1. Field of the Invention
The subject matter of this invention relates generally to fuses and more particularly to current limiting fuses of the type utilized for high voltage operation where supplemental mechanical means are utilized for low current clearing.
2. Description of the Prior Art
It has been found that current limiting fuses operate with a relatively greater or poorer degree of ease depending upon two important circuit parameters: circuit voltage and circuit current. The situation involving relatively high voltage, which may be 8.3 kilovolts or larger and relatively low overload current is particularly troublesome. Essentially the foregoing situation exists because the fusible elements usually melt at only one place along the length thereof, but the relatively low current density available after the melting operation has begun is insufficient to burn the fuse elements back fast enough to insert dielectric at a sufficient rate to cause current extinction. In the past, attempts have been made to overcome this problem using such features as special fuse fillers, element support cores constructed of out-gassing materials or combinations of out-gassing and inorganic materials, the use of auxiliary electrodes, snuffer plates, combined expulsion and current limiting fuse sections and so forth. It has been found however that these special features tend to be more successful in the lower voltage ranges than in the higher voltage ranges. Furthermore, in many instances these features introduce greater complexity to the fuse element and therefore increased cost. Also, they tend to limit the interrupting capability of the fuse at higher currents. Examples of appropriate prior art fuse constructions may be found in U.S. Pat. No. 3,256,409 issued June 14, 1966 to T. F. Brandt entitled "Current Limiting Electrical Fuse"; U.S. Pat. No. 3,840,836 issued Oct. 8, 1974 to Link entitled "Current Limiting Sand Fuse"; and U.S. Pat. No. 3,925,745 issued Dec. 9, 1975 to D. D. Blewitt entitled "High Voltage Fuse With Localized Gas Evolving Suppressors." The last-named patent is assigned to the assignee of the present invention. Other patents of interest are: U.S. Pat. No. 2,816,989 issued Dec. 17, 1957 to E. W. Sugden entitled "Electric Fuses"; U.S. Pat. No. 3,949,342 issued Apr. 6, 1976 to F. J. Kozecka entitled "Electric Fuse For Elevated Circuit Voltages"; U.S. Pat. No. 3,743,994 issued July 3, 1973 to Kozecka entitled "Ribbon-Type Fusible Element For High-Voltage Fuses And Fuse Including The Element"; U.S. Pat. No. 4,028,655 issued June 7, 1977 to R. E. Koch entitled "Electrical Current Limiting Fuse With Bound Sand Filler And Improved Low Current Fault Clearing"; U.S. Pat. No. 2,599,646 issued June 10, 1952 to F. J. Kozecka entitled "Current-Limiting Fuse"; and U.S. Pat. No. 3,825,870 issued July 23, 1974 to Y. Ono et al., entitled "Fuse Element And A High Voltage Current-Limiting Fuse." It would be advantageous therefore if a current limiting fuse could be found which operated successfully at relatively high voltage and relatively low overload current, where the fuse was relatively simple and inexpensive and where the interrupting capability at relatively high current was not compromised.